In a memory device such as PCRAM, a drift may occur as time elapses. The drift may indicate a phenomenon whereby information of a memory cell, for example, a resistance value has changed.
When a drift occurs, information read from a memory cell may differ from the original information from when the information was read. Thus, it is not easy to recover the original data. In particular, in the case of a memory device storing multi-bit data, a drift may make it more difficult to recover data. Furthermore, when a crossover occurs between adjacent levels while a drift occurs, it becomes more difficult to recover data.